The present invention relates generally to the field of conductor insulation and, more particularly, to a method for determining or predicting remaining life of conductor insulation systems. This method uses a computerized analytical model and supportive equipment to assess margin between an electrical insulation sample's given (i.e., known) void size and density and a void size and density at the threshold to integrity failure. The margin is then correlated to remaining life.
Voids and gaseous cavities originate in conductor insulation systems through a variety of mechanisms, including normal as well as improper manufacturing processes, severe or cumulative mechanical and environmental stresses, and thermal exposure.
As explained in more detail below, when a conductor insulation system, which is subjected to an electric field of sufficient magnitude, contains voids or cavities of a sufficient size and density, electrical discharges occur therein. These discharges result in an increase in current flow through the insulation between the conductor and ground or between two adjacent conductors, and a consequent reduction in the amount of current which is able to be transmitted through the conductor(s).
Further, the presence of voids or cavities of a sufficiently large size within a conductor insulation system (when subjected to an electric field or potential of sufficient magnitude) facilitates the initiation and growth of "electrical trees," which also detrimentally affect the insulation system's conductivity and projected life.
An electrical tree consists of a number of tiny hollow channels that extend and propagate from voids or impurities present in a conductor insulation system. The hollow channels can contain or allow considerable unstable discharges which, in time, may initiate further tree growth until cable failure occurs.
As explained above, when voids are present in a conductor insulation system, the insulating walls in effect "erode" in time and cause dielectric breakdown. Three processes cause this dielectric breakdown: (1) bombardment of the void's walls by ions and electrons produced when gases within the void become ionized (i.e., corona breakdown); (2) heat generated by the corona breakdown process; and (3) chemical reactions within the void, due to the formation of ozone.
A number of conductor insulation aging models have attempted to correlate various age-related factors, but they have not been entirely successful. However, a number of accepted relationships have been deduced from these aging models: (1) as applied electric field frequency increases, insulation life decreases; (2) as conductor insulation exposure time to moisture increases, insulation life decreases and the applied voltage necessary to cause insulation breakdown decreases; and (3) as residual stresses (thermal, electrical, mechanical, environmental) in conductor insulation increases, insulation life decreases.
Conventionally, manufacturers have been responsible for testing conductor insulation systems, such as cables, for obvious voids (i.e., manufacturing defects of a magnitude sufficient to allow immediate or premature cable insulation failure) before shipping the cables from the factory. The manufacturers' favored production test (i.e., the partial discharge test) is performed to cable specifications prepared by the Association of Edison Illuminating Companies (AEIC). The AEIC has only promulgated specifications for medium and high-voltage cable; currently, there are no specifications for low-voltage (i.e., 600 to 2000 Volts) cable, which is the most commonly used cable today.
The partial discharge test measures the discharge magnitude (Q), which is measured in pico-coulombs. Manufacturers are required to maintain the discharge magnitude below a specific level (i.e., not greater than 5 pico-coulombs) before shipping the cable.
The industry has accepted the partial discharge test because it uses the simplest measurement that can be made to date, and it can detect insulation degradation. However, the results of a partial discharge test indicate only whether the insulation is acceptable at that instant in time and does not provide any indication as to how long the insulation will remain acceptable. Moreover, because of the many cable failures that occur due to voids being present in cable, often after installation and over time, it is evident that the partial discharge test and other existing testing methods are not able to predict time to failure.
In addition, the integrity of conductor insulation systems can be tested by transmitting high-frequency signals down the length of a cable (i.e., from an end that has been de-terminated) and analyzing the reflected signal on an oscilloscope. While this testing methodology provides information on the condition of downstream electrical conditions, the only information it provides on the cable itself is whether a short or open circuit condition exists.
Recently, the Electric Power Research Institute (EPRI) has sponsored the development of a testing methodology that takes advantage of the generally-accepted premise that a cable's mechanical properties (e.g., hardness or elongation retention) degrades before the cable's electrical properties (e.g., dielectric strength). This testing approach is called the Ogden/EPRI Polymer Aging Monitor (Indenter) and is used to monitor cable degradation resulting from heat and radiation. Although generally effective, this testing methodology includes the following limitations: (1) the cable jacket is tested for hardness, which is a less direct (but conservative) testing method compared to testing the integrity of the conductor insulation itself; (2) repeated hardness tests (which are a mild form of destructive testing) may, in fact, lead to premature hardness and unnecessarily conservative indications of remaining life unless the area being monitored is carefully controlled and tracked to prevent repeated testing; (3) the potential for water treeing in cables with voltages up to 5 kV is not detected; and (4) because it is based on mechanical properties of cables, instead of electrical properties, the test results tend to be unnecessarily conservative.
At present, no reliable method exists to detect voids or other defects in installed conductor insulation systems, and use those results to estimate system condition and predict remaining life. Consequently, the conventional approach to evaluating cable insulation condition is by a "post-mortem" analysis after the cable has failed.
Because the presence of voids can ultimately lead to conductor insulation system failure, those industries, such as the nuclear power and aircraft industries, where the integrity of power and control systems implemented by conductor insulation systems is critical, have long desired a method for assessing insulation integrity degradation (primarily related to void size and density), confirming continued acceptable margin in insulation life, and predicting insulation remaining life, in new and installed (aged) conductor insulation systems.